Wrought Iron

The Details of a Technological System

by Don Friedman on February 20, 2018


This is an underside view of the heel connection of a heavy timber truss. The piece of wood at the top of the picture is the bottom chord, roughly 12 inches by 12 inches in section, and you can’t see the top chord above it. The bolt ties the two chords together, but the real connection between them is the birds-mouth seat for the top chord in the top surface of the bottom chord.

The part that really grabs my eye is the tapered washer that allows the diagonally-placed bolt to bear against the flat underside of the lower chord. The washer has the form of a cylinder of wrought iron with one end cut perpendicular to the cylinder’s main axis and the other cut at an angle. Assuming the bolt has been installed at the correct angle – and with the use of these washers, there is only one correct angle – the washer will only fit one way and then the nut goes on the end. It’s worth noting that this truss was built in the 1880s, at which time the standardization of screw and bolt threads was still new.

The use of those washers eliminated a triangular cut into the bottom chord to create a diagonal face of wood for the nut to bear on. This cut was difficult to make, as the angle had to be exact and making relatively delicate cuts with a saw big enough to cut the heavy timber is a pain. That cut also reduced the section of the bottom chord, weakening it in an area of high stress, but that’s not the reason that these washers were created.

The history of developments in structural technology is, to a large degree, a history of replacing skilled labor on site with more complex fabrication off-site, usually in factories. Those tapered washers used factory fabrication in wrought iron to replace the skill of a carpenter making the no-longer needed cut. If the washers were used as templates to drill the bolt hole at the right angle, as they may well have been, they also replaced some of the skill required for drilling the bolt holes.

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